This invention relates to molten aluminum, and more particularly, it relates to an improved method of heating a crucible for molten metals such as molten aluminum to provide faster heat-up times.
As noted in U.S. Pat. No. 6,049,067, incorporated herein by reference, aluminum is frequently delivered to customers in molten form. The benefits are substantial energy savings and product availability in a ready-for-use (molten) condition. Trailer mounted transport crucibles are used for this purpose. Since the heat loss from these crucibles is high, transport time is limited to a few hours, and considerable superheat must be added to the metal to ensure delivery at minimum acceptable temperature. It is common practice to heat molten aluminum to temperatures above 1700xc2x0 F. for the purpose of adding sufficient superheat. Direct impingement gas fired burners are used for this purpose, but this method is very inefficient.
Further, as noted, high temperature is undesirable because the resulting increase in metal oxidation rate generates skim. Melt loss can exceed 10%. Further, metal quality rapidly deteriorates because hydrogen solubility in aluminum is an exponential function of temperature, and oxides are formed. Refractory life is reduced by high temperature, and wall accretions build up and limit crucible metal capacity. The hazards associated with handling molten aluminum increase significantly with elevated temperature.
Another problem with molten metal such as molten aluminum involves transferring molten metal to and from the container or crucible because this requires the control of metal flow rate. The flow rate control is needed for operating, quality and safety considerations. The conventional means for controlling metal flow rate, for example, from a ladle by gravity includes varying the area available for metal flow. That is, when an orifice is positioned in the bottom of a ladle the size of the area of the orifice is changed to change the molten metal flow rate. Conventional means used to change the orifice area include a tapered rod or sometimes a slide gate. However, these provide no means for molten metal flow rate other than by varying the orifice area. Thus, when the ladle is full of molten metal, there is great force on the orifice, which when opened results in metal splashing and a hazardous situation. Further, there is an increase in oxides and reduced metal quality, particularly with molten aluminum, which readily oxidizes.
There is a great need for an improved heating method which results in savings in energy costs required for heat-up.
It is an object of the invention to provide an improved container for molten metal.
It is another object of the invention to provide an improved heat-up method for molten metal in a container.
These and other objects will become apparent from the specification, drawings and claims appended hereto.
Another embodiment of the invention contemplates a method of heating a body of molten aluminum in a container to more efficiently add heat to the molten aluminum. The method comprises the steps of providing a container having a body of molten aluminum therein, the body having a surface and the container having a bottom and applying heat to the body. A pipe or conduit is provided in the body, the pipe having a first portion thereof adjacent the bottom and at least one opening thereinto to permit molten aluminum to flow into the pipe, the pipe having a second portion thereof adjacent the surface having at least one opening in the second portion to permit molten aluminum to flow out of the pipe into the body at or near the surface. Gas is introduced to the pipe to flow molten aluminum upwardly therein, the molten aluminum flowing into the pipe through the opening in the first portion and out of the pipe through the opening in the second portion. In this method, molten aluminum in the container is circulated to provide a stirring motion to more efficiently add heat to the body of molten aluminum. That is, for example, in this method cold molten metal on the bottom of the container can be transferred to or near the surface for surface heating and molten metal circulated in this manner to avoid or minimize temperature stratification in the container. Thus, heat is transferred by mixing rather than by conduction which leads to heating the metal at the surface far beyond the target temperature.
The invention also includes an improved container for containing molten metal which may be employed to maintain the molten metal at target temperature longer.